-LRB- CNN -RRB- -- A timeline of events leading to the April 20 explosion on the Deepwater Horizon oil rig in the Gulf of Mexico , according to documents from congressional investigators and testimony from witnesses to the disaster :

^ 12:35 a.m.

Oilfield services contractor Halliburton completes cementing on BP 's Mississippi Canyon 252 well . In the hours leading up to midafternoon , crews conduct three positive pressure tests , in which fluid weight is placed on the seal .

^ 11 a.m.

BP and Transocean executives argue over changes in the drill plan for the well at a meeting aboard the rig .

^ 5:05 p.m. .

An unexpected loss of fluid is observed in the riser pipe , suggesting there are leaks in the blowout preventer , a critical piece of equipment designed to shut down the well in the event of an emergency .

^ 5-7 p.m.

Negative pressure tests , in which crews reduce fluid pressure to test for leaks through the cement or well casing , show unexpected results . One draws 15 barrels of liquid out of the well instead of the expected five , raising more concerns about leaks . In a second , pressure on the rig 's `` kill line '' -- a high-pressure pipe used to cut off the flow of oil -- falls to zero , while rising to 1,400 pounds per square inch in the drill line , indicating a buildup of natural gas .

^ 8 p.m.

BP ends testing and proceeds to replace remaining drill fluids with seawater . A BP investigator later suggests this is a `` fundamental mistake . ''

^ 9:00 p.m. .

More fluid is reported flowing out of the well than is being pumped in .

^ 9:10 p.m.

Well pump is shut down for a `` sheen '' test , but the well continues to flow . Drill pipe pressure unexpectedly increases .

^ 9:30 p.m.

Abnormal pressures and more fluid returns are observed . The well pump is abruptly shut down .

^ 9:50 p.m.

Gas surges from the well and up the riser . The supply ship Damon B. Bankston , which is tied up alongside Deepwater Horizon , reports drilling fluid spilling onto its deck and is told to move back 500 meters from the rig . Seconds later , the first explosion occurs .

^ 9:52 p.m.

Deepwater Horizon issues distress call . The order is given to abandon the rig . Eleven of the 126 on board are lost at sea when the platform sinks two days later .

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Documents from Congress , witness testimony detail events before leak

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At one point , BP and Transocean executives argue over changes in the drill plan

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Unexpected loss of fluid suggests leaks in the blowout preventer